The nature of the amine storage complexes in vesicles of platelets from various species has been studied utilizing the technique of nuclear magnetic resonance. The molecular mobility of intra-vesicular serotonin appears to be a unique characteristic of each species, suggesting that wide variation is possible in the nature of the complex. In human platelets, extra-vesicular amine also appears to have characteristic chemical and motional properties. With the use of ring-fluorinated amines, it is now possible to study more critically the nature of vesicular amine storage in nerve microsacs.